NATURALLY OCCURRING ARSENIC IN GROUNDWATER AND ITS EFFECT ON BENTHIC LIFE IN STREAMS

Over the last several years, the North Carolina Department of Environment and Natural Resources' Division of Water Quality (DWQ), Aquifer Protection Section, in cooperation with the NC Geological Survey, NC State University, and the US Geological Survey, has been conducting research into the occurrence and effects of naturally occurring arsenic in North Carolina. Arsenic is a metallic element often associated with sulfide minerals, and is especially prevalent in rocks of the Carolina Slate Belt. Arsenic precipitates out of groundwater at the oxidation-reduction boundary. The precipitates can occur as a coating on the sediments at the sediment-water interface of streams.

The focus of this study is groundwater-surface water interaction and the potential to explain surface water biological and chemical impairments. There are many stream segments in North Carolina that are not supporting their natural use, yet the cause of impairment is unknown. We tested the hypothesis that benthic fauna living in and digesting sediment may be stressed by the presence of arsenic in the sediment coatings. Personnel of DWQ's Environmental Sciences Section carried out a benthic macroinvertebrate assessment and a sediment aquatic toxicology assessment at a site in the eastern Carolina Slate Belt. The results showed that the benthic macroinvertebrate community was comparable to reference conditions, but toxicity testing of surficial sediments indicated significant levels of acute toxicity to laboratory test organisms.